HAT-P-18b and HAT-P-19b: Two Low-Density Saturn-Mass Planets Transiting Metal-Rich K Stars

We report the discovery of two new transiting extrasolar planets. HAT-P-18b orbits the V=12.759 K2 dwarf star GSC 2594-00646, with a period P=5.508023±0.000006 d, transit epoch Tc=2454715.02174±0.00020 (BJD), and transit duration 0.1131±0.0009 d. The host star has a mass of 0.77±0.03 Msun, radius of 0.75±0.04 Rsun, effective temperature 4803±80 K, and metallicity [Fe/H]=+0.10±0.08. The planetary companion has a mass of 0.197±0.013 Mjup, and radius of 0.995±0.052 Rjup yielding a mean density of 0.25±0.04 g cm-3. HAT-P-19b orbits the V=12.901 K1 dwarf star GSC 2283-00589, with a period P=4.008778±0.000006 d, transit epoch Tc=2455091.53417±0.00034 (BJD), and transit duration 0.1182±0.0014 d. The host star has a mass of 0.84±0.04 Msun, radius of 0.82±0.05 Rsun, effective temperature 4990±130 K, and metallicity [Fe/H]=+0.23±0.08. The planetary companion has a mass of 0.292±0.018 Mjup, and radius of 1.132±0.072 Rjup yielding a mean density of 0.25±0.04 g cm-3. The radial velocity residuals for HAT-P-19 exhibit a linear trend in time, which indicates the presence of a third body in the system. Comparing these observations with theoretical models, we find that HAT-P-18b and HAT-P-19b are each consistent with a hydrogen-helium dominated gas giant planet with negligible core mass. HAT-P-18b and HAT-P-19b join HAT-P-12b and WASP-21b in an emerging group of low-density Saturn-mass planets, with negligible inferred core masses. However, unlike HAT-P-12b and WASP-21b, both HAT-P-18b and HAT-P-19b orbit stars with super-solar metallicity. This calls into question the heretofore suggestive correlation between the inferred core mass and host star metallicity for Saturn-mass planets.